I'm starting to think we might have this all wrong. The mural depicts a Xenomorph as well as facehugger (lower corner). It looks more and more to me that the Xenomorphs were almost worshiped or honored. I think the connection between the Engineers and the Xenomorphs is more important than our connection to the Engineers.
What if we were created to breed Xenomorphs and that was it? Engineers realized that the Xenomorphs need a host to grow and develop into these "weapons". Instead of using fellow Engineers they created humans who shared the same DNA and similar body structure. The perfect host to breed more Xenomorph weapons without having to use Engineer bodies. One Engineer sacrifices himself to create humans, humans are sacrificed to create Xenomorphs, Xenomorphs are sacrificed in presumably war and the cycle continues. The theme of sacrifice comes full circle. So far that's the best theory I can come up with.
Another note is the cave paintings... We gotta assume that all Engineers have their own personalities like humans. What if Earth really was just a breeding ground (of many) and an Engineer who decided humans didn't deserve to be sacrificed tried to hint humans to go to the "military installation" and stop the destruction of Earth? I might be reaching haha.
Not my photo or my find.